feat: tách luồng âm instrument theo môi trường — docker dùng FluidSynthWASM, standalone dùng native pyfluidsynth (+Carla VSTi)

- runtime: detect()/capabilities() trả environment (docker|standalone)
- vst_engine: render_soundfont_midi_to_audio (pyfluidsynth native, event-stream, release tail)
- plugins: POST /soundfont-render — render MIDI notes -> WAV (auth, fallback default SF)
- frontend: SonicRuntime.environment + dataset.environment; app.jsx route preview/transport:
  standalone+SF -> soundfont-render WAV (playNativeSfNote/scheduleNativeSfItem),
  docker giữ WASM, VSTi standalone giữ Carla bridge

app/static/js/services/api.js

@@ -89,6 +89,7 @@ window.API_BASE_URL = window.API_BASE_URL || window.location.origin;
         // Render MIDI notes → WAV bằng VSTi (pedalboard, âm thật) — trả file_id
         // để UI preview / gán clip vào track / download
         midiRender: (payload) => apiRequest('/api/v1/plugins/midi-render', { method: 'POST', body: JSON.stringify(payload) }),
+        soundfontRender: (payload) => apiRequest('/api/v1/plugins/soundfont-render', { method: 'POST', body: JSON.stringify(payload) }),
         // Phát dãy MIDI notes realtime qua Carla bridge (OSC) — preview khi
         // pedalboard không render được plugin (VD VST2)
         carlaPlayNotes: (payload) => apiRequest('/api/v1/plugins/carla-play-notes', { method: 'POST', body: JSON.stringify(payload) }),